Motor cores are generally composed of many layers of thin metal plates that are stacked on top of each other to form a laminated stack. Typically, the laminated stack is fastened with welds, rivets, or bolts which hold and/or compress the layers together to permanently maintain the stack in a fixed orientation. Stator windings are then fitted about or wound in place about grooves or slots formed in the inside peripheral surface of the laminated stack to form poles of the motor. During processing of the metal plates and prior to assembly, the plates are annealed to form an oxide layer on the surfaces of each metal plate. The oxide layer effectively insulates one plate from the adjacent plate, provided that the plates are not subject to significant compression. Use of rivets or bolts that compress the stack "shorts-out" the oxide layer causing some or all of the metal layers to be electrically coupled to adjacent layers, essentially "short circuiting" the stack forming a conductor. Welding the layers together similarly creates a short circuit between the plates. In some motor applications this is acceptable, and even desirable.
However, in other motor applications, this is not acceptable, as shorted plates reduce the efficiency of the motor by increasing eddy current loss in the stack. In such applications, the metal layers must be electrically isolated from adjacent layers. Such stacks are referred to as "loosely laminated" stacks because the metal plates are not subject to significant compressive force. Typically, applications requiring a loosely laminated stack are directed toward smaller motors, such as fractional horsepower motors in the range of one-half to one horsepower. However, some loosely laminated stack motors may be as large as five horsepower. The loosely laminated stack must be fixed so as to prevent the metal plates from becoming skewed while simultaneously avoiding detrimental compressive force.
It is known to use clamps to hold the plates in position while the windings are attached or wound about the slots or grooves in the stack. This is costly and labor intensive, and care must be taken not to apply too much compressive force. Application of too much compressive force results in shorting some or all of the laminations, while application of too little compressive force permits the plates to move, resulting in air gaps between the laminations and skewed laminations. Accordingly, use of clamps is disadvantageous in the manufacturing of loosely laminated stacks.
In some applications, large clamping or compressive force is used in conjunction with a chemical adhesive. Of course, a loosely laminated core cannot be manufactured in this way. Such methods use slow-curing adhesives that require the core to remain under compression for relatively long periods of time while the adhesive hardens. Application of such compressive force may involve expensive and bulky fixtures and is an inefficient use of manufacturing floor space. Also, such methods using adhesive are disadvantageous if large compressive force is not used. Without use of substantial compressive force, the cores may suffer from lack of rigidity and lack of squareness if the glue is not permitted to harden, undisturbed, for a relatively long period of time. Therefore, compression of the core is required during this time to insure dimensional accuracy. As described above, such compression causes shorting between the layers, thus this method cannot be used to produce loosely laminated cores.
It is also known to provide a cylindrical bore through the stack that is filled with a chemical adhesive, which when dry, bonds the layers together. Again, this is expensive and time consuming. In known methods, it is difficult to keep all of the metal layers aligned. Failure to maintain alignment between the metal layers results in "skew," which severely reduces the efficiency of the motor, thus affecting motor performance. Skewed motor cores are unacceptable.